This invention relates generally to electrical contacts on SiC-based semiconductor devices, and more particularly to ohmic and rectifying contacts on silicon carbide semiconductors.
SiC possesses tremendous advantages for high temperature and high power solid state electronics. In addition, it offers potential advantages for high frequency and logic circuit applications: e.g., power conversion (mixer diodes, MESFETs), single chip computers (n-MOS, CMOS, bipolar transistors), non-volatile random access memory SiC CCDs can hold charge for more than a thousand years thus, for example, making hard disks a thing of the past.
The potential maximum average power, maximum operating temperature, thermal stability, and the reliability of SiC electronics far exceeds Si or GaAs based electronics. The degree to which these advantages of SiC can be utilized, however, is presently constrained by the thermal stability and electrical properties of the metal/SiC junctions. The primary reasons for this are: (1) the power density of SiC devices is limited by the thermal stability of the ohmic contact junctions; and (2) substantial cooling is required to insure the stability of electrical contact junctions.
For a long time, researchers have been striving without success to develop electrical contacts to silicon carbide trying overcome these constraints. Extensive review of prior work is disclosed in an article by Porter, Lisa M. and Davids, Rober F, entitled xe2x80x9cA critical review of ohmic and rectifying contacts for silicon carbide,xe2x80x9d Materials Science and Engineering B 34 (1995) 83-105. Until these constraints are removed, SiC devices/circuits offer only marginalxe2x80x94if anyxe2x80x94advantages over Si and GaAs. Utilization of the full performance potential of SiC itself (for all devices), requires four types of performance-limiting electrical contacts: (1) ohmic to p-type -SiC, (2) ohmic to n-type SiC, (3) rectifying to p-type SiC, and (4) rectifying to n-type SiC.
The value of SiC electronics lies in its potential to extend the capabilities of solid state electronics beyond what is possible with Si or GaAs. Thus, suitable electrical contact characteristics obtained in the laboratoryxe2x80x94under low stress conditionsxe2x80x94must not drift or degrade, due to changes at the metal/SiC junctions, under actual device operating conditions. This requires two additional attributes of metal/SiC electrical contacts. First, the contact metal must form a junction with SiC that is chemically stable to approximately 1000xc2x0 C. joule heating at high forward current densities in power SiC devices could easily cause metal/SiC junctions to reach such temperatures). Second, the contact metal (or metallization structure) must act as a diffusion barrier to circuit and bonding metals (electrode metals) at the same temperatures. Metal/SiC electrical contacts demonstrated previously do not come close to meeting all these stability requirements.
The best known p-type SiC ohmic contact metallizations demonstrated to date are Al, Al/Ti and Pt (Refs. 2,3). Although specific contact resistances as low as 1xc3x9710xe2x88x925 xcexa9xc2x7cm2 have been reported in laboratory studies, actual SiC devices exhibit p-type contact resistances of 1xc3x9710xe2x88x922 xcexa9xc2x7cm2 to 1xc3x9710xe2x88x923xcexa9xc2x7cm2 (Refs 10, 11).
Aluminum and its silicides, and Al/Ti form thermally unstable interfaces with SiC and/or have melting problems (Ref 12, pg. 2). Platinum reacts with SiC to form many different suicides at temperatures as low as 280xc2x0 C. At temperatures as low as 400xc2x0 C., Pt continues to react with SiC until it is entirely consumed by the formation of PtSi (Refs. 4, 5). Thus, Pt cannot protect its contact from circuit and bonding metal diffusion; this characteristic of Pt, in contact with SiC, requires that Pt itself not be used as a circuit or bonding metal. Further, PtSi reacts with virtually all suitable circuit and bonding metallizations at very low temperatures.
Virtually every electronic devicexe2x80x94unipolar or bipolarxe2x80x94requires n-type ohmic contacts. The most successful n-type SiC ohmic contact metallizations demonstrated to date are Ni and TiC. Both exhibit specific contact resistances between 1xc3x9710xe2x88x925 xcexa9xc2x7cm2 and 1xc3x9710xe2x88x926xcexa9xc2x7cm2 (Ni: Ref 13; TiC: Refs. 7and9). Nickel forms silicides at very low temperatures. Its graded Ni-silicide junction is thermally unstable, and it cannot form a protective diffusion barrier to circuit and bonding metals. Other Ohmic contacts formed of silicide, nitride, carbide or multiple layers of such materials exhibit stability problems similar to Ni contacts. Transition metals such as Mo and W do not adhere well to SiC surfaces, and they spall at elevated temperatures.
TiC forms an electrical ohmic contact junction with n-type SiC, that is stable to at least 1400xc2x0 C. (Ref. 14). However, it reacts with all candidate circuit and bonding metals at low temperatures; thus, it cannot form a protective diffusion barrier to circuit and bonding metals.
TaC contacts to doped SiC have also been described in a paper by Jang, T., Rutsch, G. W. M., Odekirk, B., Porter, L. M., entitled xe2x80x9cA comparison of single- and multi-layer ohmic contacts based on tantalum carbide on n-type and osmium on p-type silicon carbide at elevated temperatures,xe2x80x9d in International Conference on Silicon Carbide and Related Materials, Session: SiC Devices, (abstract submitted May 2000) and an article by Jang, T., Porter, L. M., entitled xe2x80x9cElectrical characteristics of Tantalum and Tantalum Carbide Schottky diodes on n- and p-type silicon carbide as a function of temperature,xe2x80x9d Transaction of the Fourth High Temperature Electronics Conference, Albuquerque, N.M., (June 1998). These papers suggest that TaC forms a suitable electronic contact to SiC but suffer similar practical limitations or drawbacks to TiC contacts. In particular, a need remains to protect the contact system from oxidation.
Low work function metals and semi-metals, that do not react with SiC to form tunnel junctions, can be used to form rectifying electrical contacts to p-type SiC. This type of electrical contact has not been required for SiC devices demonstrated to date; thus, there is little, if any, background literature. We have demonstrated that TiC forms an excellent rectifying contact to p-type SiC, that is stable to at least 1400xc2x0 C. (Ref 14.). However, it reacts with all candidate circuit and bonding metals at low temperatures; thus, it cannot form a protective diffusion barrier to circuit and bonding metals.
The n-type, rectifying Schottky diode is required to modulate the current and voltage in all majority carrier solid state devices. The most important devices of this type are Schottky diodes and MESFETs. Excellent adhesion and abrupt metal-SiC interfaces are cornerstone requirements for this type of contact. Metallizations previously developed for this purpose exhibit various deficiencies. For example, attempts to make thermally stable Schottky rectifying junctions on n-type SiC have been stymied. Many of the metals that are capable of forming such junctions (e.g., Ti, Au/Ti, Pt and PtSi) react with SiC to form silicides at relatively low temperatures, orxe2x80x94in the case of Auxe2x80x94do not adhere to the SiC surface. The electrical contact junction is thermally unstable, leading to compositional grading of the junction interface at elevated temperatures. This grading effect is exacerbated by the creation of free carbon at the original metal/SiC interface, causing substantial performance degradation, and limiting SiC device operation to well below the capabilities of SiC itself. The (W/Ti, Ti, Au/Ti and Pt)/SiC n-type Schottky junctions demonstrated by Cree Research, N.C. State, NASA Lewis and the Japanese, are unstable at high temperatures or under sustained high current density conditions.
The best rectifying contact metal to n-type SiC, demonstrated to date appears to be W/Ti, extensively tested at the U.S. Army Res. Labs in Adelphi, Md. This contact, however, exhibited unstable leakage-current characteristics, probably due to thermally-driven reactions at the interface. The testing consisted of cycling the junctions between room temperature and 500xc2x0 C. (number of cycles unknown), and measuring the junction reverse leakage current I(L) at room temperature as a function of the number of cycles. During a first series of cycles, I(L) increased. During the next series of cycles, I(L) decreased, reaching a minimum value about 10 xcexcA greater than it was before stress testing was begun. Thereafter, I(L) did not change.
U.S. Pat. No. 2,918,396 to Hall describes formation of rectifying and ohmic contacts on silicon carbide PN diodes and PNP and NPN transistors by alloying two silicon-acceptor or silicon-donor alloy globules to an N-type or P-type silicon carbide crystal at high temperature, (1700xc2x0 C.). The alloying metals disclosed include aluminum, phosphorus, tungsten, molybdenum or tungsten-molybdenum, with nickel or tungsten conducting electrodes.
U.S. Pat. No. 3,308,356 to Rutz discloses a method of making rectifying contacts to PN junctions in a silicon carbide substrate bonded on one side to a tungsten block by alloying fragments of silicon doped with Ga (P-type) or As (N-type) onto the exposed face of the SiC substrate in a forming gas atmosphere (90% N, 10% H) at high temperature.
U.S. Pat. No. 3,510,733 to Addamiano discloses forming on SiC semiconductor devices electrical leads of an alloy consisting primarily of chromium and nickel, but which can include traces of Si, C and Fe.
U.S. Pat. No. 4,738,937 to Parsons describes a method of making nonrectifying (ohmic) Schottky contacts on a semiconductor substrate, such as Si or SiC, by epitaxially depositing a metal having suitable work function and lattice parameters, the identified metals including Yb on Si, NiSi2 and W on N-type Si, and TiC on N-type xcex2-SiC.
U.S. Pat. No. 5,442,200 to Tischler discloses forming an ohmic contact on a SiC surface by depositing a sacrificial silicon layer followed by a metal (Ni, Cr, Pd, Ti, W, Ta, Mb, Co, Zr or mixtures or alloys thereof to form a noncarbonaceous ohmic contact structure.
U.S. Pat. No. 5,448,081 to Malhi proposes a MOSFET device formed on a SiC substrate, with doped source and drain regions but does not disclose any means for connecting electrodes to such regions.
U.S. Pat. No. 5,471,072 to Papanicolaou discloses a Pt rectifying contact and a Ti/Au ohmic contact on n-type SiC. Both forms of contacts degrade at high operating temperatures, e.g., subject to catastrophic degradation at over 800xc2x0 C. as disclosed and probably unstable at temperatures of 500xc2x0 C. or less.
Although the semiconductor materials, a and xcex2-SiC, have a demonstrated capability for stable, efficient performance at high temperatures, the same is not true of SiC devices/circuits, due to the instabilities of their electrical contact structures.
Also, applicant has learned that a TiC contact on SiC does not, by itself, form a diffusion barrier to circuit or bonding metals. Appropriate circuit/bonding metals such as W, Pt, Au and Pd, form intermetallics with TiC. These solid state reactions change the composition of the electrical contact junction, thus degrading it.
Accordingly, a need remains for thermally stable ohmic and rectifying electrical contacts to n-type and p-type SiC.
It is, therefore, one object of the invention to make an electrical contact to SiC; where, the contact metal exhibits the following properties:
1. Metal/SiC junction stable to over 1000xc2x0 C.,
2. Protects its junction with SiC from circuit and bonding metal diffusion (forms an electrically transparent diffusion barrier) to  greater than 1000xc2x0 C., and either
3. Forms an ohmic electrical contact to p-type SiC, or
4. Forms rectifying (Schottky) contact to n-type SiC.
Another object of the invention is to form an electrically transparent diffusion barrier (ETDB) for TiC electrical contacts to SiC; where, the ETDB exhibits the following properties:
1. ETDB/TiC junction is stable to over 1000xc2x0 C.,
2. Protects its junction with TiC from circuit and bonding metal diffusion to 1000xc2x0 C.; thereby, also protecting the TiC/SiC junction.
In one aspect of the invention, I have discovered that osmium (Os) forms a rectifying (Schottky) metal junction on n-type SiC semiconductor surfaces, which remains abrupt and firmly attached to at least 1050xc2x0 C. A thermally stable contact structure is made by forming a metallic osmium layer on the surface of a semiconductive SiC substrate (either xcex2 or xcex1 ) and connecting a conductive metal electrode to the osmium layer through a suitable metal bonding and protective layer, such as Pt or PtAu alloy. The room temperature rectifying characteristics of this structure on n-type SiC are essentially unchanged by annealing at temperatures as high as 1050xc2x0 C. for 2 hours in an inert environment (Ar, hydrogen or vacuum). These junction degrade, but remain operable after annealing for 1 hour at temperatures as high as 1175xc2x0 C. The resulting barrier height of Os/n-type SiC junctions is 1.78xc2x10.1 ev. Of equal importance, the Os layer forms a diffusion barrier, which prevents diffusion of electrode metals to the Os/(n-type SiC) junction.
On P-type SiC, the Os layer will form an ohmic contact with the lowest specific contact resistance of now possible with ohmic contacts to p-type SiC. Stable, low resistance, p-type ohmic contacts are the key to development of all SiC bipolar high temperature power electronics. Forming an Os contact on p-type SiC will provide a lower-energy tunneling barrier and thinner tunnel depth than Pt contacts on p-type SiC.
Os can also be deposited on SiO2 to form MOS gate structures and field spreading structures. Os on SiO2 forms a stable interface and has mechanical and thermal properties equal to those of Os on SiC.
Another aspect of the invention, is a method and structure for making an electrically transparent diffusion barrier (ETDB) which shields TiC/SiC junctions from electrode metals to 1150xc2x0 C., and which forms a metallurgical junction with TiC that remains firmly attached and is stable to 1150xc2x0 C. The ETDB structure consists of 2 layers: (1) a tungsten carbide (WC) layer in contact with the TiC surface, and (2) a layer of elemental tungsten (W) on the WC surface. This ETDB structure works because W is stable in contact with WC at a temperature greater than 1150xc2x0 C. The TiC/SiC junction remains stable because W does not react with WC to form W2C in this temperature range (unlike all other transition metals in contact with their mono-carbides). Thus, a concentration gradient to drive diffusion between the WC/TiC, and thus, TiC/SiC interfaces does not exist. 1.78Elemental W is the actual diffusion barrier. It forms a bond with potential electrode metals because of solid solubility which increases with increasing temperature. Once xe2x80x9cformedxe2x80x9d, the thin bonding layer is absolutely stable at or below the forming temperature. Electrodes can then be freely connected to this contact layer either directly or though a suitable intermediate bonding layer such as Pt or PtAu alloy. Such contacts are stable to at least 1050xc2x0 C.
The invention enables fabrication of SiC devices and circuits that will not degrade, or become unstable, under all strain conditions (thermal, electric field, mechanical) for which SiC itself can perform as a semiconductor device. This invention removes all restrictions imposed by metal contacts on the operating envelope of SiC solid state device technology. In particular, it provides rectifying (Schottky) junctions and ohmic contacts that will withstand sustained exposure to temperature as high as 1150xc2x0 C., and electron migration effects at high electric fields.
Os forms a rectifying Schottky junction to n-type SiC, as do several other metals. Os is set apart from all other metals by the following characteristics: (1) it forms an electrically transparent diffusion barrier to 1050xc2x0 C. {which protects the junction from diffusion of electrode metals}, (2) the Os/SiC junction itself remains stable to 1050xc2x0 C., and (3) the electrical properties of the junction do not change after sustained exposure to temperatures as high as 1050xc2x0 C. Os will form an ohmic contact junction to p-type SiC exhibiting the same unique stability characteristics as the n-type SiC rectifying junction.
W/WC is an electrically transparent diffusion barrier, which forms a stable junction with TiC to 1150xc2x0 C. Its purpose is to protect the chemical and electrical integrity of the TiC/SiC junction. TiC forms ohmic contact junctions to n-type SiC; and rectifying contact junctions to p-type SiC. The TiC/SiC junction, by itself, is stable to temperatures greater than 1400xc2x0 C.
Applicant has further discovered that W/WC in combination with TaC forms an effective contact structure on either n-type or p-type SiC. W/WC layers form a high integrity interface to the TaC/SiC contacts. The TaC layer is protected from oxidation. The W/WC TaC/SiC structure is chemically stable. For example, a W/WC/TaC contact on SiC is shown to be stable for at least 1000 hours at 600xc2x0 C. and also stable for at least 400 hours at 1000xc2x0 C. Specific contact resistance is shown to be less than 1xc3x9710xe2x88x925 xcexa9xc2x7cm2.
Based on the foregoing developments, applicant has developed a family of high temperature/power semiconductor devices which satisfy a number of unmet commercial needs. One immediate application is thermal sensors. The invention can be used to make junction or resistive thermistors, that measure temperatures to 1922xc2x0 F. ( greater than 1050xc2x0 C.), and 2100xc2x0 F. (1150xc2x0 C.). This development permits SiC thermistors to compete, for the first time, with thermocouples, oxide resistors and pyrometers in the temperature range of 300xc2x0 F. to 2100xc2x0 F. (150xc2x0 C. to 1150xc2x0 C.). It also enables the manufacture of hostile environment transducers, operable at temperatures above 460xc2x0 F. (238xc2x0 C.).
The invention enables making Schottky diodes, PN diodes and transistors as either discrete devices or as components of integrated circuits. Importantly, the invention opens up many potential applications in the power conditioning and conversion field, such as power rectifiers for converting AC power to clean DC power. N-SiC Schottky rectifiers could handle powers nearly as high as the best Si PN junction rectifiers, and PN junction SiC rectifiers could replace vacuum tubes. Other types of devices include bipolar transistors, thyristors, MOSFETs, MESFETs, IGBTs, and mixer diodes for use in communications, as well as digital and analog to digital electronics.
In summary, all solid state semiconductor devices require one, or more, of the four electrical contacts discussed, which the present invention makes possible.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.